OSPF State Confusion

[lowfell]

From the following output of #show ip ospf neighbor  it say the state is FULL/DR

Which router is the DR here?

Is it Router 2 or it's neighbor 192.168.45.1 ?

If it is Router2 then because it says DR is it fare to say that it CAN'T be a point to point ?

Router2# show ip ospf neighbor 

Neighbor ID     Pri    State      Dead Time    Address     Interface
192.168.45.1    1      FULL/DR    00:00:36     10.0.0.1    Ethernet0

[Jon Marshall]

It is the other router that is the DR.

Not sure what R2 being the DR or not has to do with point to point status ?

As a general answer a physical point to point connection could still have a DR/BDR election unless under the interfaces you configured -

"ip ospf network point-to-point"

and then you would not.

Jon

[lowfell]

So you are saying that   Router1 is the DR ? & not Router2 ?

If router one is the DR then the Network type can't be point to point because point to point DO NOT ELECT DR BDR is this statement correct as well?

[Jon Marshall]

I am now you have said the neighbor is Router 1 (you didn't say in your original post :))

The point to point network type does not elect a DR/BDR so correct in what you say.

The point I was trying to make is that you can have a physical point to point link but OSPF still sees it as a broadcast network so it would elect a DR/BDR unless you tell OSPF not to.

Jon

[lowfell]

The point I was trying to make is that you can have a physical point to point link but OSPF still sees it as a broadcast network so it would elect a DR/BDR unless you tell OSPF not to.

Sorry Jon. that doesn't make sense to me. Everything I've read says that an ospf point to point link DOES NOT ELECT DR/BDR

like this document for example

http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13687-15.html

Please be patient with me, I'm just trying to understand it.

[Jon Marshall · Accepted solution]

No problem, I think I have probably confused the issue.

So with an OSPF network type of point to point there is no election of a DR/BDR so everything you have read is correct and so are you.

But I am talking about physical links not OSPF network types.

Imagine you connect two L3 switches to each other and make that a routed L3 connection at either end using a /30 subnet ie. the only two devices on that subnet are the two switches.

So physically that is a point to point link but not to OSPF it isn't because to OSPF it is a broadcast network type which just so happens to have only two devices on it so a DR/BDR election would take place.

If you then configured "ip ospf network point-to-point" under the physical interfaces on each of the switches then that tells OSPF to treat it as a point to point link and then there is no election.

Like I say I may well have just confused the issue so apologies if I have.

Does that make sense ?

Jon

[lowfell]

Thanks Jon. I've got it now! Much appreciated.